Various types of wiring substrates are used for electronic devices such as personal computers and smartphones. Among the wiring substrates, multilayer wiring substrates manufactured by a build-up method especially have a high wiring density and can contribute to the performance improvement of the electronic devices.
In the multilayer wiring substrate, wiring layers and insulating layers are alternately laminated. In view of thinning the multilayer wiring substrate, it is preferable to thin the insulating layer. Moreover, one of the insulating layers is used as a dielectric layer for a capacitor in some cases. In this case, thinning the dielectric layer can increase the electrostatic capacitance of the capacitor.
Note that, the following documents disclose techniques related to the present application: Japanese Laid-open Patent Publication No. 2004-235323; Japanese Laid-open Patent Publication No. 2005-44833; International Patent Publication No. WO 2006/016589; Japanese Laid-open Patent Publication No. 2006-173544; Japanese Laid-open Patent Publication No. 2004-140254; Japanese Laid-open Patent Publication No. 2007-123797; and Japanese Laid-open Patent Publication No. 2012-33968.
However, thinning the dielectric layer is actually difficult due to various reasons.
For example, in the case where a prepreg in which glass fiber is impregnated with an epoxy resin is used for a dielectric layer, the thickness of the glass fiber itself becomes a factor which prevents the dielectric layer from being thinned, so that it is difficult to form a dielectric layer having a thickness of 30 μm or less.
Moreover, even in the case where an epoxy resin containing no glass fiber is used for a dielectric layer, it is difficult to insulate upper and lower wiring layers from each other with the dielectric layer, when the surface of the underlying wiring layer is rough, unless the dielectric layer is formed to have a sufficient thickness. For example, a wiring layer formed by the plating method has a surface roughness larger than 0.3 μm due to crystal grains. Therefore, unless a dielectric layer is formed to be thicker than 10 μm for safety, there is a possibility that upper and lower neighboring wiring layers cannot be insulated from each other with the dielectric layer.